1. Field of the Invention
The present invention generally relates to wellbore completion. More particularly, the invention relates to downhole tools. More particularly still, the invention relates to an expandable bit with a pressure activated release member.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. The drill bit generally includes a body portion for securing the drill bit to the drill string and a crown portion to form the wellbore. After drilling a predetermined depth, the drill string and the drill bit are removed, and the wellbore is lined with a string of steel pipe called casing. The casing typically includes a smaller outside diameter than the drill bit that formed the wellbore. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. The casing typically extends down the wellbore from the surface of the well to a designated depth. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is typically filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore.
In a conventional completion operation, it is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a second designated depth of a smaller diameter, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing and then cemented in place. This process is typically repeated with additional casing strings until the well has been drilled to a total depth.
The process of cementing a liner into a wellbore typically involves the use of wiper plugs and drill-pipe darts. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore. A liner wiper plug is typically located inside the top of a liner, and is lowered into the wellbore with the liner at the bottom of a working string. The liner wiper plug has radial wipers to contact and wipe the inside of the liner as the plug travels down the liner. The liner wiper plug has a cylindrical bore through it to allow passage of fluids.
Typically, the cementing operation requires the use of two plugs and two darts. When the cement is ready to be dispensed, a first dart is released into the working string. The cement is pumped behind the dart, thereby moving the first dart downhole. The first dart acts as a barrier between the cement and the drilling fluid to minimize the contamination of the cement. As the first dart travels downhole, it seats against a first liner wiper plug and closes off the internal bore through the first plug. Hydraulic pressure from the cement above the first dart dislodges the first dart and the first plug from the liner and pumped down the liner together. At the bottom of the liner, the first plug seats against a float collar, thereby closing off fluid flow through the float collar. The pressure builds above the first plug until it is sufficient to cause a membrane in the first plug to rupture. Thereafter, cement flows through the first plug, the float collar, a shoe track, and a float shoe and subsequently into the annular space between the liner and the wellbore.
Generally, the shoe track is a space defined between the float collar and the float shoe. The shoe track is used to ensure that the float shoe is surrounded in high quality cement and that any contamination or lower quality cement that may bypass a second liner wiper plug is safely contained within the shoe track.
After a sufficient volume of cement has been placed into the wellbore, a second dart is deployed. Drilling mud is pumped in behind the second dart to move the second dart down the working string into the liner. The second dart travels downhole and seats against a second liner wiper plug. Thereafter, hydraulic pressure above the second dart dislodges the second dart and the second plug from the liner and they are pumped together down the liner. In turn, cement ahead of the second plug is displaced through the shoe track and subsequently out of the liner into the annulus. After the cementing operation is complete, the shoe track is typically drilled out as the well is drilled to another designated depth.
From time to time and for a variety of reasons it is necessary to form a portion of a wellbore that is at least as large as the section of the cased wellbore thereabove. For example, a monobore well consists of a sequence of expandable liners that are run through the existing casing, then expanded to achieve the same post-expansion through-bore. In forming the monobore well, the portion of the wellbore below the cased portion must be at least as large as the section of the cased wellbore thereabove.
There are a variety of different methods of forming an enlarged wellbore. One such method is by positioning a conventional under-reamer behind a drill bit to cut the enlarged wellbore. In this drilling configuration, the drill bit acts as a pilot bit to cut the inner cross-sectional area while the under-reamer enlarges the cross-sectional area. Generally, the conventional under-reamer includes a number of expandable arms that move between a closed position and an open position. The ability of the conventional under-reamer to open and close the arms allows the under-reamer in the closed position and the pilot bit to travel though a smaller diameter casing. After passing through the casing and the shoe track the under-reamer may be opened to form an enlarged diameter bore below the casing shoe resulting in a wellbore equal to or larger than the original drilled hole. Thereafter, the enlarged wellbore may be lined with expandable liners. This procedure of forming the enlarged borehole, although effective may be time consuming and expensive.
In recent years bi-center bits have been developed as an alternative to the conventional under-reamer. Generally, the bi-center bit includes offset cutting members mounted at irregular intervals around the crown of the bit. As the bi-center bit is rotated, the offset cutting members rotate to form an enlarged wellbore. Although, this method of forming an enlarged wellbore is becoming more common the bi-center bits are unstable due to their irregular structure and tend to be more difficult to control for directional purposes than ordinary drill bits. Additionally, the bi-center bits may not drill the expected swept diameter of the offset pads which ream the pilot hole created by the crown.
More recently, an expandable bit has been used to form an enlarged portion of the wellbore. The expandable bit was introduced to overcome the deficiencies in the conventional under-reamer and the bi-center bit. An example of an expandable bit is disclosed in International Publication Number WO 01/81708 A1, which is incorporated herein in its entirety. Similar to the conventional under-reamer, the expandable bit includes a set of blades that move between an open position and a closed position. Generally, in operation hydraulic fluid is pumped through the center of the expandable bit to move the blades between the open and the closed position. A more detailed discussion of the expandable bit will be described in subsequent paragraphs.
Even though the expandable bit overcomes many of the deficiencies in the conventional under-reamer and the bi-center bit, a problem still exists with the use of the expandable bit to drill out the shoetrack. The problem occurs when the blades in the expandable bit inadvertently move from the closed position to the open position and contact the ID of the casing while drilling out the shoetrack, thereby resulting in an increase of torque and damage to the blades.
In view of the deficiency of the expandable drill bit, a need therefore exists for an expandable bit with a pressure activated release member to selectively allow the blades to move from the closed position to the open position. There is a further need for an improved expandable bit.